JPH0542611A - Crosslinked polyolefin-based resin foam - Google Patents

Abstract

PURPOSE:To provide crosslinked polyolefin-based resin foam excellent in formability by a method wherein the difference between the physical properties in the flow direction and in the width direction of foamed sheet is small and the drawdown at heat forming is suppressed. CONSTITUTION:The crosslinked polyolefin-based resin foam concerned is obtained by crosslinking foamable resin composition sheet containing polyolefin- based resin and heat decomposition type foaming agent under the condition that the foam is stretched at least to one direction in the crosslinked foam expanded by heating and, at the same time, the relationship between the expansion ratio and the thickness, widths and lengths of the sheets before and after expansion satisfies the specified relation.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polyolefin resin crosslinked foam, and more specifically, there is little difference in the physical properties between the flow direction and the width direction of a foam sheet, and the drawdown during thermoforming is suppressed. The present invention relates to a polyolefin resin crosslinked foam having excellent properties.

[0002]

2. Description of the Related Art Conventionally, polyolefin resin cross-linked foams have been used for a wide range of applications such as heat insulating materials, cushioning materials, and sundries, and recently, as interior materials for vehicles such as automobiles, ceilings, doors, It is widely used as a heat insulating material for instrument panels.

These heat insulating materials and the like are generally commercialized by processing a sheet-shaped polyolefin resin crosslinked foam (foamed sheet) into a predetermined shape by a thermoforming method such as vacuum forming or pressure forming.

The thermoforming method generally includes a step of heating a foamed sheet, a step of molding the foamed sheet with a force of vacuum or compressed air or a mold, a step of cooling the foamed sheet, and cutting an extra portion from a molded product. It consists of the process of doing. In the step of heating the foamed sheet, the foamed sheet held between the clamps is heated and softened, but during this heating, a drooping phenomenon (drawdown) occurs due to the weight of the foamed sheet. Depending on the crosslinked foamed polyolefin resin, the drawdown may be so severe that molding may not be possible, the molded product may be wrinkled, or the thickness may become non-uniform and the commercial value may be impaired.

On the other hand, recently, a foam molded article having a complicated and deep shape has been required, and a high degree of deep drawing has been required. However, for deep drawing, it is necessary to heat the foamed sheet to a considerably high temperature,
As a result, drawdown becomes more severe and thermoforming becomes more difficult.

Further, the polyolefin resin crosslinked foam is obtained by molding a resin composition containing a polyolefin resin and a thermal decomposition type foaming agent into a foamable resin composition sheet at a temperature lower than the decomposition temperature of the foaming agent, and After cross-linking, foaming is performed by heating. Foaming methods include salt bath foaming, horizontal hot air foaming,
There are vertical hot air foaming, etc., but the management of these foaming processes is
The expansion ratio, the thickness of the pre-foaming sheet (foamable resin composition sheet), the design of the width, the foaming temperature, etc. are mainly used. Therefore, the flow direction of the foam (M
In many cases, there are differences in the physical properties between D) and the width direction (TD).

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to reduce the difference in the basic physical properties depending on the direction of the foam and the drawdown at the time of thermoforming to a minimum, and to improve the molding processability. To provide a foam.

As a result of intensive studies to overcome the problems of the prior art, the inventors of the present invention have drawn a foamed sheet near the end of foaming in at least one direction, and have a foaming ratio and a thickness of the sheet before and after foaming. By controlling the relationship between the width and the length within a certain range, there was little difference in the physical properties of the foam in the flow direction (MD) and the width direction (TD), and drawdown was significantly suppressed. The inventors have found that a foamed sheet can be obtained, and have completed the present invention based on the findings.

[0009]

According to the present invention, in a crosslinked foamed product obtained by crosslinking a foamable resin composition sheet containing a polyolefin resin and a thermal decomposition type foaming agent and heating and foaming the foamed resin composition sheet. When the body is stretched in at least one direction and the relationship between the expansion ratio and the thickness, width and length of the sheet before and after expansion is defined by the following formulas [1] to [4], the constants A, B and C are as follows. There is provided a polyolefin resin crosslinked foam characterized by satisfying the relationship of

0.5 ≦ A <1.0 0.5 ≦ B <1.0 1.1 ≦ C <2.5 Formula [1] H = (T ₂ / T ₁ ) × (W ₂ / W ₁ ) × (L ₂ / L ₁ ) Formula [2] T ₂ / T ₁ = A × ³ √H Formula [3] W ₂ / W ₁ = B × ³ √H Formula [4] L ₂ / L ₁ = C X ³ √H However, A × B × C = 1 In addition, the meaning of each symbol is as follows. H: Expansion ratio T ₁ : Thickness of foaming sheet T ₂ : Thickness of foaming sheet W ₁ : Width of foaming sheet W ₂ : Width of foaming sheet L ₁ : Length of foaming sheet L ₂ : Foaming sheet Length of

The present invention will be described in detail below. In the manufacturing process of the polyolefin resin cross-linked foam, first, a thermal decomposition type foaming agent in the polyolefin resin, and if necessary, an antioxidant, a cross-linking aid, a pigment, etc. are blended, and the temperature is lower than the decomposition temperature of the foaming agent. The mixture is melt-kneaded at a temperature and formed into a sheet by an extruder or the like (single screw extruder, twin screw extruder, pressure kneader, etc.).

The foamable sheet thus obtained is crosslinked by irradiation with ionizing radiation (α-ray, β-ray, γ-ray, electron beam, etc.).
For the cross-linking, a chemical cross-linking method using an organic peroxide, a water cross-linking method by grafting or copolymerizing vinyltrimethoxysilane or the like with a resin, and the like may be adopted.

The crosslinked foamable sheet is heated to a temperature above the decomposition temperature of the foaming agent to foam. As the foaming method, there are various methods such as a method of passing through a preheating area and a foaming area in a foaming furnace by using a vertical blowing furnace or a horizontal foaming furnace of a hot air circulation type, or a salt bath method.

In the present invention, the heat-foamed foamed sheet is stretched in at least one direction. Examples of the stretching treatment method include a method of enlarging the sheet width so as to have a predetermined width in the vicinity of the foaming end point (width increasing step).

To expand the width of the foamed sheet, for example, a method of sucking both ends of the foamed sheet with a suction type guider, a method of pulling both ends of the foamed sheet with tenter, or a width direction outside of the foamed sheet with a circular guider or a roll type guider is used. Examples include a method of applying a contact frictional force to one side. This widening processing can be performed in one step or in multiple steps. Also, 1
The temperature of the foamed sheet may be lowered after the stage-widening process and then the second-stage widening process may be performed.

As the stretching treatment, there is a method of stretching the foamed sheet in the machine direction by changing the speed between rolls when the foamed sheet is taken up. Furthermore, these methods can be combined.

By such a stretching treatment, the foamed sheet is stretched in at least one direction with respect to complete free foaming (foaming in which no stretching treatment is carried out in the longitudinal direction, the transverse direction and the thickness direction).

In the present invention, by performing such a stretching treatment, the foamed sheet defines the relationship between the expansion ratio and the thickness, width and length of the sheet before and after foaming according to the above formulas [1] to [4]. Constants A, B and C
Control so as to satisfy the above relationship.

Note that A × B × C≈1. Also, L ₂
/ L ₁ is a length of 50 cm in the length direction of the sheet before foaming
The line (L ₁ ) is drawn with a felt-tip pen or the like, the length (L ₂ ) of the line is measured after foaming, and the ratio is calculated.

When any one of the constants A, B and C does not satisfy the above relation, the difference in the physical properties of the foamed sheet in the length direction and the width direction becomes very large, and at the time of thermoforming. In the heating step, the drawdown of the foamed sheet becomes large.

The reason for this is not clear at this stage, but a foamed sheet satisfying the above conditions will undergo one kind of annealing when the foamed sheet is heated to the softening point in the heating step during thermoforming. It is presumed that this is because the amount of hanging due to the weight of the foam sheet is absorbed. Therefore, if even one of the above conditions is not satisfied, the linear expansion cannot be completely absorbed, or the length direction,
The difference in the physical properties in the width direction becomes very large, and the molding process becomes extremely difficult.

In order to control the foaming process so as to satisfy these relational expressions, in each foaming method, not only the expansion ratio, the thickness and width of the pre-foaming sheet, the foaming temperature, etc. It is desirable to conduct an experiment on the relevance of the above relational expression with.

The polyolefin resin used in the present invention includes low density polyethylene (LDPE), high density polyethylene (HDPE) and linear low density polyethylene (L).
-LDPE), polypropylene (PP), ethylene-vinyl acetate copolymer (EVA) and the like can be used, but not limited thereto.

Examples of the thermal decomposition type foaming agent include azodicarbonamide (ADCA), oxybenzenesulfonyl hydrazide, dinitrosopentamethylenetetramine and the like. The addition amount thereof is appropriately adjusted according to the desired foaming ratio, but is usually a resin. It is about 2 to 25 parts by weight with respect to 100 parts by weight.

If desired, a crosslinking agent such as trimethylolpropane trimethacrylate (TMP), a flame retardant, a coloring agent, an antioxidant (such as a phenolic antioxidant), a filler, a foaming aid, a lubricant, etc. may be added. May be.

[0026]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.

[Example] A resin composition having a formulation shown in Table 1 was extruded with a twin-screw extruder to obtain a width of 400 mm and a thickness of 1.0 m.
m sheets were obtained. After irradiating this sheet with an electron beam having a dose shown in Table 1 to crosslink it, the sheet was introduced into a hot air circulation horizontal foaming furnace, preheated to about 155 ° C. by hot air from the upper and lower surfaces, and from there to the foaming region. It was transferred and heated and foamed by hot air at 230 ° C.

The foamed sheet was sucked at both ends of the foamed sheet with a suction type guider disposed adjacent to the foaming furnace. The expansion ratio is adjusted by the number of parts added with the foaming agent and the heating amount of the heater,
The width was adjusted by the position of the suction type guider, and the length was adjusted by the sheet feeding and take-up speed. The results are shown in Table 2.

[0029]

[Table 1]

[0030]

[Table 2]

<Evaluation of Drawdown> 70 × 7 foam
It was cut to 0 cm, held by a clamp, and heated from above and below by a heater until the surface temperature reached 160 ° C. At that time, the foam droops due to its own weight, but the drooping length (mm) of the largest part of the droop at the center of the foam sheet was measured. As a result, the hanging length is 10
When the distance was within mm, it was evaluated as ◯, and when it exceeded 10 mm, it was evaluated as x.

[0032]

According to the present invention, there is provided a foam having a small difference in physical properties depending on the flow direction of the foam, little drawdown, and excellent moldability.

Continuation of front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location B29K 23:00 105: 04 B29L 31:00 4F C08L 23:00

Claims (1)

[Claims] 1. A crosslinked foam obtained by crosslinking a foamable resin composition sheet containing a polyolefin resin and a thermal decomposition type foaming agent, and heat-foaming the foam, wherein the foam is stretched in at least one direction, and When the relationship between the expansion ratio and the thickness, width and length of the sheet before and after expansion is defined by the following formulas [1] to [4], constants A, B and C
Satisfying the following relations: a polyolefin resin crosslinked foam. 0.5 ≦ A <1.0 0.5 ≦ B <1.0 1.1 ≦ C <2.5 Formula [1] H = (T ₂ / T ₁ ) × (W ₂ / W ₁ ) × ( L ₂ / L ₁ ) Formula [2] T ₂ / T ₁ = A × ³ √H Formula [3] W ₂ / W ₁ = B × ³ √H Formula [4] L ₂ / L ₁ = C × ³ √ However, A × B × C = 1 and the meaning of each symbol is as follows. H: Expansion ratio T ₁ : Thickness of foaming sheet T ₂ : Thickness of foaming sheet W ₁ : Width of foaming sheet W ₂ : Width of foaming sheet L ₁ : Length of foaming sheet L ₂ : Foaming sheet Length of